Off-stream blending reservoirs allow community water systems dependent on a river for their water supply to store water of good quality that can be used for blending during periods of poor quality or during droughts. Reservoir managers must decide the maximum concentration of a contaminant that they will choose to pump into the reservoir. Using a simplified representation of the off-stream blending reservoir system in Pontiac, Illinois, an approach for estimating the firm yield under the following three different operation rules is presented: (1) the system's current rule in which the reservoir is only filled when the nitrate-N concentration in the Vermilion River does not exceed 4 mg/L, (2) an alternative rule in which the reservoir can be filled as long as the reservoir concentration does not exceed 4 mg/L and (3) a rule in which inflows cannot occur if the reservoir concentration exceeds 4 mg/L or if the inflow exceeds a given threshold. These estimates can be made using a model with a daily time step during a period from 1979 to 1999 for which data for both streamflow and nitrate-N concentrations in the river were available. The sensitivity of the firm yield to nitrate sinks in the reservoir can be evaluated for each operating rule. An ensuing publication will contain the results of this research.